CCD image sensors and CMOS image sensors are conventionally known as solid-state imaging elements for converting image light from a subject into an electric signal. Among them, the CMOS image sensors include a plurality of light receiving sections (a plurality of photodiodes) for generating a signal charge by light irradiation, and an amplifying transistor (MOS transistor) for reading the signal charge generated at the light receiving sections as an imaging signal, both of which are provided on a common substrate. The CMOS image sensors thus have advantages of low power consumption, low cost performance by the use of standard CMOS processing technique such as system LSI, and multiple usages.
In such CMOS image sensors, with the tendency of the downsizing of pixels in recent years, it is important to secure the sensitivity in low illumination intensity. Patent Literature 1 discloses a conventional solid-state imaging element equipped with high sensitivity pixels and low sensitivity pixels.
FIG. 8 is a plan view of a partial imaging region, illustrating a pixel arrangement for improving the sensitivity at low illumination intensity in the convent ional solid-state imaging element disclosed in Patent Literature 1.
As illustrated in FIG. 8, the conventional solid-state imaging element disclosed in Patent Literature 1 includes pixels with a large area indicated by uppercase letters, R, G and B, as well as pixels with a small area indicated by lowercase letters, r, g and b, i.e., high sensitivity photoelectric conversion elements and low sensitivity photoelectric conversion elements, arranged checkerwise. When imaging signals from adjoining high and low sensitivity photoelectric conversion elements of the same color are synthesized and obtained, a synthesized signal can be regarded as a signal from a single pixel positioned at the centroid of the two pixels (which is indicated by a dot “•” in FIG. 8), resulting in image signals in a virtual checkerwise arrangement. A pixel with a large area and a pixel with a small area of the same color are paired and they are arranged in a Bayer arrangement. Therefore, the dynamic range can be expanded without substantial lowering of resolution in a lengthwise direction and a widthwise direction.
In the CMOS image sensors, it is generally known to use color filters of three primary colors, such as R, G and B (Red, Green and Blue respectively), in synthesizing images. This is introduced in Patent Literature 2, and the like.
FIGS. 9(a) to 9(c) are each a plan view of an essential part, illustrating by stages a color filter forming step of a conventional solid-state imaging element disclosed in Patent Literature 2.
First, as illustrated in FIG. 9(a), a G filter 201 is formed on a pixel region 200, which is shared by four pixels, in such a manner that G (green) is arranged among the Bayer arrangement of the three primary colors, R, G and B. Next, as illustrated in FIG. 9(b), an R filter 202 is formed in such a manner that R (red) is arranged among the Bayer arrangement of the three primary colors, R, G and B. Then, as illustrated in FIG. 9(c), a B filter 203 is formed in such a manner that B (blue) is arranged among the Bayer arrangement of the three primary colors, R, G and B. As a result, a pixel array of 2×2 units (shared by four units) is formed, which is arranged in a two-dimensional Bayer arrangement.
Patent Literature 1: Japanese Laid-Open Publication No. 2004-336469
Patent Literature 2: Japanese Laid-Open Publication No. 2000-294758